linux-kernel - Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches

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Message-ID: <874l9z31c5.fsf@intel.com>
Date:   Wed, 23 Jan 2019 16:17:30 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>
Cc:     dev@...nvswitch.org, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        netdev@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
        linux-security-module@...r.kernel.org,
        kernel-hardening@...ts.openwall.com,
        intel-wired-lan@...ts.osuosl.org, linux-fsdevel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, Laura Abbott <labbott@...hat.com>,
        linux-kbuild@...r.kernel.org,
        Alexander Popov <alex.popov@...ux.com>
Subject: Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches

On Wed, 23 Jan 2019, Greg KH <gregkh@...uxfoundation.org> wrote:
> On Wed, Jan 23, 2019 at 03:03:47AM -0800, Kees Cook wrote:
>> Variables declared in a switch statement before any case statements
>> cannot be initialized, so move all instances out of the switches.
>> After this, future always-initialized stack variables will work
>> and not throw warnings like this:
>> 
>> fs/fcntl.c: In function ‘send_sigio_to_task’:
>> fs/fcntl.c:738:13: warning: statement will never be executed [-Wswitch-unreachable]
>>    siginfo_t si;
>>              ^~
>
> That's a pain, so this means we can't have any new variables in { }
> scope except for at the top of a function?
>
> That's going to be a hard thing to keep from happening over time, as
> this is valid C :(

Not all valid C is meant to be used! ;)

Anyway, I think you're mistaking the limitation to arbitrary blocks
while it's only about the switch block IIUC.

Can't have:

	switch (i) {
		int j;
	case 0:
        	/* ... */
	}

because it can't be turned into:

	switch (i) {
		int j = 0; /* not valid C */
	case 0:
        	/* ... */
	}

but can have e.g.:

	switch (i) {
	case 0:
		{
			int j = 0;
	        	/* ... */
		}
	}

I think Kees' approach of moving such variable declarations to the
enclosing block scope is better than adding another nesting block.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center